Well, the simple truth is adding efficient compression to the row storage (which is what heap does) is not really possible. Or more precisely - you can do that outside the database by using a filesystem with a compression (like zfs), and doing that within the database won't give you much advantage. Which is probably why no one really proposed implementing that, because the cost/gain ratio is just not worth it.

The problem with heap and compression is that it mixes a lot of different data (i.e. data from different columns), which is not great for compression ratio. To address that, it's necessary to use some variant of a columnar format, and allowing such stuff is one of the goals of the table AM API, zedstore AM etc.